*{
    margin: 0;
    padding: 0;
}
#btn {
    width: 40px;
    height: 40px;
    position: fixed;
    right: 14px;
    bottom: 30px;
    background: url('../images/goto-top .png') no-repeat left top;
}
#btn:hover {
    background:url('../images/goto-top .png')  no-repeat  left -40px;
}